There is a widespread use of an image forming apparatus utilizing electrophotographic technology which employs toner to develop an electrostatic latent image formed on a photoreceptor and transfers the developed image on a sheet of paper. Specifically, such an image forming apparatus as an on-demand printing apparatus required to provide higher image quality and higher resolution has come to utilize the wet development method employing a liquid developer with toner having a diameter smaller than that in a dry developer.
In recent years, a proposal has been made of liquid developing apparatuses using a high-viscosity liquid developer wherein a solid toner made of resin and pigment is dispersed, in high concentration, in a liquid carrier which is an insulating liquid such as silicone oil.
When a liquid developer is used for development, it is preferred that a thin layer of liquid developer of the order of microns be formed on the developer carrying member such as a developing roller, and this thin-layered liquid developer should be brought in contact with a photoconductor. This arrangement is particularly preferred when high-viscosity liquid developer is employed.
To stabilize the image density, a very thin layer of liquid developer is required to be uniformly formed on the developing roller. One of the techniques proposed so far for forming this thin layer of developer is a method of using a coating roller wherein the liquid developer on the coating roller is applied onto the developing roller under predetermined conditions. Further, a technique of using a supply roller is proposed to supply a predetermined volume of liquid developer to the coating roller.
To be more specific, a supply roller is used to supply a predetermined volume of liquid developer to the coating roller, and a thin layer of liquid developer is formed while the developer is conveyed by the coating roller, and then, the developer is transferred onto the developing roller. The developing roller brings a thin layer of liquid developer in contact with the photoconductor, whereby an electrostatic latent image is developed on the photoconductor.
When the liquid developer is transferred from roller to roller, the problem is in the behavior of the liquid developer on both ends of the roller in the longitudinal direction (across the width). Thus, it is important to determine whether the liquid developer is to be transferred on the entire surface across the roller or is to be transferred on both ends of the roller across the width with a margin of safety.
For example, when the liquid developer is to be transferred from a narrower (shorter) roller A to a wider (longer) roller B, a line-shaped area, where a lot of liquid developer exists, is formed the part, of the roller B, opposed to both ends of the roller A—i.e., both ends of the developer layer to be supplied. This is caused by the liquid developer wrapping around the end faces on both ends of the roller A, or the liquid developer wrapping around the end faces on both ends of the roller A, in the nip portion between the roller A and roller B, by surface tension.
If the liquid developer is transferred to another roller with the liquid developer unevenly formed, the unevenness still remains on that roller, with the result that the unevenness will remain uncorrected at the time of development.
Conversely, when the liquid developer is to be transferred from a wider (longer) roller B to a narrower (shorter) roller A, an uneven area does not occur to the transferred liquid developer. However, the liquid developer on the surface of the roller B spreads to the end faces of both ends of the roller A and sticks there. The liquid developer sticking to the end faces of both ends of roller A is spread to reach the shaft portion of the roller A by the repeated rotation/stop operations of the roller A. It is further spread in the axial direction, and the liquid developer eventually reaches the bearing section and leaks out of the development apparatus or enters the bearing section, thereby causing troubles.
To avoid this problem, it may be possible to provide both ends of the roller with a sealing member for preventing the developer from spreading. However, provision of many rollers with sealing members will complicate the apparatus structure and will increase the cost.
The Japanese Patent No. 3521975 discloses an apparatus wherein the liquid developer is sequentially transferred using a plurality of coating rollers. This apparatus is structured in such a way that the width of the coating roller is gradually reduced. However, for many rollers in this structure, there is required some measures such as a seal member in order to prevent the liquid developer from wrapping around the end faces on the ends of the rollers.
In the Japanese Unexamined Patent Application Publication No. 2006-72334, the width of the developing roller is smaller than that of the coating roller (measuring roller). Further, use of a plurality of measuring rollers is also mentioned, but this is based on the same concept as that of the aforementioned Japanese Unexamined Patent Application Publication No. 3521975. Thus, similar problems remain to be solved.
According to the technique disclosed in the Japanese Unexamined Patent Application Publication No. 2006-243050, an anilox roller is used as the coating roller, and the width of the anilox roller pattern, namely, the width of the liquid developer supply area is designed smaller than that of the developing roller. Since a liquid developer is not transferred to the entire surface of the developing roller, the liquid developer on the developing roller may be uneven.
According to the technique described in the Japanese Unexamined Patent Application Publication No. 2000-235306, the width of the coating roller is greater than that of the developing roller, and the width of the area wherein the liquid developer is coated by the coating roller is smaller than that of the developing roller. In this arrangement, the width of the liquid developer to be transferred is smaller than that of the developing roller. This may cause the liquid developer on the developing roller to be uneven.